The technology of pumping lasers has evolved substantially in recent years and it is now becoming possible to have a pulsed laser source which gives an average pumping power in the order of a hundred or so watts and a thermal deposition in the crystal of the same order of magnitude.
Various solutions have been implemented to resolve the problem of extraction of the thermal energy in the crystal:                either the crystal is cooled in a conventional manner by a circulation of water on its periphery which is an effective solution but does not make it possible to produce systems operating at a hundred or so watts and above,        or the crystal is cooled by using a cryogenic system which increases the thermal conductivity of the crystal to enhance the cooling thereof and reduce the focusing effects linked to the thermal charge in the crystal induced by the optical pumping.        
These configurations exhibit drawbacks, notably in terms of costs, footprint and vibrations.
In the lasers of high average power (>10 watts), these solutions are not satisfactory and a rear face cooling is used.
An exemplary multipass amplification device configuration implementing a cooling of the amplifying medium 1 by a rear cooling (illustrated by the arrow) is illustrated in FIGS. 1a and 1b. The beam to be amplified 2 arrives on the amplifying medium 1 with an angle of incidence φi according to the front view (FIG. 1a) and θi according to the view from above (FIG. 1b). This device is multipass by virtue of the fact that the beam 2 passes several times through the amplifying medium 1 by virtue of the mirrors 3. The rear face 11 of the amplifying medium is reflecting to reflect the beam toward the amplifying medium. The amplified beam 6 leaves in the direction Oy, and can be seen only in FIG. 1a. In these figures, the beam is represented by a single ray.
The cooling can be obtained by means of a fluid, liquid or gas, or a solid. This cooling by the rear face makes it possible to increase the heat exchange surface area. Moreover, it makes it possible to generate a thermal gradient in the direction of propagation of the laser in the crystal, and thus to achieve a high thermal extraction. The index variations linked to the temperature variations in the crystal are gradients mostly oriented in the same direction as the direction of propagation of the laser beam. However, there then arises a problem of temporal contrast which was not encountered in the preceding solutions. The temporal contrast is defined by the ratio between the intensity of the main pulse and the foot of the pulse and/or any parasitic pulses.
Consequently, there is still, today, a need for an amplification device that simultaneously gives satisfaction to all the abovementioned requirements, in terms of cooling and of temporal contrast.